Microwave ovens have become increasingly popular in recent years due in large part to the speed with which a conventional microwave oven can cook certain foods. Microwave ovens produce high frequency, electromagnetic energy fields which cause certain molecules to oscillate at a greater rate thereby producing heat. For example, a water molecule has a dipole which absorbs microwave energy and indirectly converts that microwave energy to thermal energy. The heat produced by the interaction of microwave energy and water molecules is generally not greater than about 100.degree. C. because the water evaporates at that point. Many food substances comprise sufficient quantities of water, or other microwave absorbing materials, to make them susceptible to microwave cooking.
In a conventional oven, electricity, gas, wood, etc. is converted to thermal energy. The thermal energy is transmitted to the air within the oven, the oven walls, the oven racks, the food being cooked and the container the food is being cooked in. Additionally, conventional ovens operate by heating the outside of the food being cooked and wherein the interior portion of the food being cooked is heated by the conduction of thermal energy from the exterior surface of the food to the interior. Cooking food from the exterior surface inward is both slow and inefficient because, as mentioned briefly above, the entire interior of the oven and all the contents of the oven must be heated. However, conventional ovens, while slow and generally energy inefficient, have one perceived advantage over conventional microwave ovens. Because the thermal energy envelopes the exterior of the food being cooked it is often possible to "brown" and/or "crisp" the exterior of the food product. It has heretofore been difficult to brown or crisp food in a conventional microwave oven.
Microwave ovens, which typically operate at 2450 MHZ, supply microwave energy which is absorbed by the "lossy" component of foods. A lossy component is any portion of food, or other product, which absorbs microwave energy and converts at least a portion of that microwave energy to thermal energy. Microwave ovens are typically designed so that the microwave energy is not absorbed by the interior surfaces of the microwave. Thus, microwave energy does not generally heat the interior surfaces of the microwave oven. While the microwave cooking process is energy efficient, the exterior of the food product is typically cooked at the same rate as the interior of the food. Thus, browning and/or crisping of the food's exterior generally does not occur in a microwave oven.
There have been many attempts to rectify this shortcoming of microwave ovens, i.e., to brown and/or crisp food while cooking it in a microwave oven. For example, U.S. Pat. No. 5,493,103, which issued on Feb. 20, 1996 to Kuhn, discloses a baking utensil which essentially surrounds the food being cooked with a layer of material containing ferrite particles. The ferrite particles absorb microwave energy, and convert it to thermal energy until the Curie temperature of the ferrite is reached. The Curie temperature is a characteristic of the ferrite and different particles can be selected depending upon their Curie temperatures and the desired cooking results. When the Curie temperature is reached the particulate ferrite layer reflects excess microwave energy away from the food.
The process described by Kuhn is inherently inefficient in that some of the microwave energy is reflected away from the food. Moreover, because the food is completely surrounded by a particulate ferrite layer, the microwave energy is not transmitted directly to the food but must generally be converted to thermal energy. The conversion of microwave energy to thermal energy essentially eliminates the benefits of microwave cooking, i.e., the speed associated with the direct absorption of microwave energy by molecules in the food being cooked. Thus, while it may be possible to shield food from microwave energy and convert the microwave energy to thermal energy, this process essentially converts the microwave oven to an inefficient conventional thermal conduction oven.
U.S. Pat. No. 5,396,052, which issued on Mar. 7, 1995 to Betcavich, et al. discloses a cooking pot having a lid wherein the base material of the pot and lid is essentially transparent to microwave energy. The interior of the cooking pot is glazed with a microwave absorbing material. The food inside of the pot is cooked by normal thermal conduction as the interior glaze both converts microwave energy to thermal energy and reflects excess microwave energy away from the food inside of the container. While this configuration may provide the desired browning and/or crisping on the exterior of the food, it does not retain the speed and energy efficiency of a conventional microwave oven.
As an alternative to completely encasing food in a microwave absorbing material, a suceptor layer, or layers, of microwave absorbing material have been used in an attempt to brown at least a portion of the exterior of food placed on the suceptor layer. In general, any material that converts microwave energy to thermal energy is considered a "suceptor". However, the term suceptor is often used to refer to a layer of microwave absorbing material. For example, U.S. Pat. No. 4,542,274, which issued on Sep. 17, 1985 to Tanonis, et al. describes a microwave cooking pan, for example a pie pan, wherein a layer of plastic with magnetic particles disbursed evenly throughout is used as a heating layer. The heating layer converts microwave energy to thermal energy thereby browning at least a portion of one surface of the food placed thereupon.
Additionally, U.S. Pat. No. 5,144,106, which issued on Sep. 1, 1992 to Kearns, et al. uses a layer of cooking oil or fat as a suceptor. The oil or fat is separated from the food being cooked by a material which can conduct heat from the oil or fat to the food. The fat or oil absorbs microwave energy, converts it thermal energy which is conducted to the layer of material between the food and the oil. Thus, a surface is provided where the food can be cooked both by thermal conduction and microwave absorption. The fat or oil produces a temperature in excess of 100.degree. C. on which to cook the food because fats and oils typically boil at a much higher temperature. A typical microwave oven can heat cooking oil or fat to a temperature of from about 125.degree. C. to 225.degree. C.
The references discussed above utilize flat, essentially continuous layers of material which absorb microwave energy, reflect microwave energy or both. Flat continuous sheets of suceptor material were generally preferred in the past to avoid the problem of "arcing" and/or localized overheating of the conductive element. Arcing, and localized overheating can burn food in the microwave oven, damage the microwave oven itself, and in extreme cases cause fires to start within the microwave oven. For example, a small pin hole in a metallic layer can cause arcing across the hole which results in sparks and/or damage to the metallic layer. Decorative utensil handles, for example forks, spoons and the like, often "arc" due to the multiple edges, layers and non-uniformities in the metallic structure. Likewise, the ends of exposed elements, for example the rods disclosed in U.S. Pat. No. 3,591,751 which issued Jul. 6, 1971 to Goltsos, can arc and/or overheat near the tips of the rods.
Attempts have been made to design suceptors which are not flat and continuous sheets of microwave absorbing material. For example, U.S. Pat. No. 5,322,984, which issued on Jun. 21, 1994 to Habeger, Jr. et al., shows a series of "antenna" elements which are used to collect microwave energy, convert it to thermal energy and transmit the thermal energy to the grill element which is in contact with the food being cooked. It has been observed that antenna configurations similar to those disclosed in the patent to Habeger Jr. et al. can cause localized areas of overheating near the antenna ends, and arcing can also occur. Additionally, the antenna configuration also requires that a substantial portion of the element be placed away from the food which results in a significant amount of heat being generated at the antennas which is not directly transmitted to the food.
Thus, there has been a continuing need for an efficient microwave grill apparatus which can convert microwave energy to thermal energy to brown and/or crisp the exterior surface of a food product. Moreover, there exists a need for a microwave grilling apparatus which can grill food while at the same time avoiding arcing and/or localized areas of overheating in the apparatus. Additionally, there is a need for a microwave grilling apparatus which maintains the benefits of microwave cooking, i.e., speed and energy efficiency while adding the desired browning and/or crisping function which has generally been lacking in conventional microwave cooking apparatuses.
Another type of cooking process which is difficult to perform in a microwave is to steam cook food. Food is steam cooked by placing the food above or in the same closed container with boiling water. As the water boils and evaporates into steam, the hot air produced thereby along with the pressure within the closed vessel acts to heat and steam cook the food. One problem with attempting to steam cook food in a microwave is that the microwave energy which is absorbed by the water to convert the water into steam simultaneously cooks the food thereby defeating the purpose of steam cooking. It would be advantageous to "shield" the food from the microwave energy yet allow the microwave energy to permeate through the tray to be absorbed by and boil the water. However, in order to further decrease the cooking time required to steam the food, it may be desirable to allow a small amount of the microwave energy to be transmitted to the food to assist slightly in cooking the food.
One device that attempts to address the problem of shielding a majority of the microwave energy while allowing a small portion of the microwave energy to be absorbed by the food is shown in U.S. Pat. No. 5,558,798 which discloses a microwave steam cooking apparatus having a bottom water tray formed of a material which is transparent to microwaves allowing the microwave energy to permeate therethrough and boil the water contained within the water tray. A base container which may be double layered is placed on top of the water tray. An outer layer of the base tray is formed of a material which reflects microwaves and an inner layer is formed of a material which is transparent to microwaves. A plurality of openings is formed in the outer layer to allow a small amount of the microwaves to permeate through the base container and be absorbed by the food. A top tray covers the base and is formed of a material which reflects microwaves. The top tray may also be formed with a plurality of openings to allow a small amount of the microwaves to travel therethrough. As the microwaves boil the water contained within the water tray, the steam rises through a plurality of vent holes formed in a bottom of the base container to steam the food. A small amount of microwave energy permeates through the openings of the base container and top tray to assist the steam in cooking the food.
Although this prior art microwave steam cooking apparatus is adequate to steam cook food, it does not allow both grilling and steam cooking food in a microwave oven without substantially modifying the apparatus.
Thus, there has been a continuing need for an efficient microwave grill and steamer apparatus which can convert microwave energy to thermal energy to brown and/or crisp the exterior surface of a food product and which also may be used to allow the apparatus to convert water into steam which is used to steam cook food. Moreover, there exists a need for a microwave grilling and steaming apparatus which can grill or steam food while at the same time avoiding arcing and/or localized areas of overheating in the apparatus. Additionally, there is a need for a microwave grilling and steaming apparatus which maintains the benefits of microwave cooking, i.e., speed and energy efficiency, while adding the desired browning and/or crisping function which has generally been lacking in conventional microwave cooking apparatuses. Further, there is the need for a microwave steaming apparatus which may be used to reflect a majority of the microwave energy to prevent the microwave energy from substantially cooking the food, but allows a small amount of the microwave energy to permeate through the apparatus to assist in cooking the food.